The prevalence of chlamydial genital infection was studied in 177 prostitutes in Iran; 100 in Teheran and 77 in the port of Bandar Abbas. Chlamydia trachomatis was isolated in eight (6.9%) of 116 patients with valid cultures. Type-specific antibodies were found against C trachomatis serotypes D to K (genital serotypes) in 94.2% and against serotypes A to C (trachoma serotypes) in 2% of the prostitutes. Type-specific IgM at a titre of greater than or equal to 8, indicating current infection, was found in 29.2%, whereas type-specific IgG at a titre greater than or equal to 64, suggesting a current or recent infection, was present in 71.5%. The lower chlamydial isolation rate in these women may have been due to previous treatment with antichlamydial drugs and because of immune responses resulting from repeated reinfection with chlamydiae. The results indicate that in Iran prostitutes are commonly infected with C trachomatis and are probably a major reservoir of chlamydial genital infection.